SOA-MZI Device Fault Isolation

ABSTRACT

A photonic integrated circuit having a plurality of circuit components, is disclosed, which may include an MMI for splitting signal power passing therethrough among first and second optical pathways coupled to first and second outputs, respectively, of the MMI, thereby directing first and second percentages of the signal power along the first and the second optical pathways, respectively; and a photodetector integrated into the photonic integrated circuit and coupled to said first optical pathway for measuring a signal power level on said first optical pathway.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.10/446,451, filed May 28, 2003, entitled “SOA-MZI Device FaultIsolation” which claims the benefit of U.S. Provisional PatentApplication 60/384,063, filed May 29, 2002, entitled “SOA-MZI DeviceFault Isolation,” both of which prior applications are incorporated byreference herein in their entirety.

TECHNICAL FIELD

This invention relates to optical communications networks. Moreparticularly, the invention relates to a method and apparatus forinternal monitoring and fault isolation in photonic integrated circuits(PICs).

BACKGROUND OF THE INVENTION

Photonic integrated circuits (PICs) are, in the general sense,integrated devices comprising passive components such as waveguides andmulti-mode interferometers (MMIs), as well as active components such asSemiconductor Optical Amplifiers (SOAs).

While certain single-active optical devices such as lasers are availablewith back facet monitors, PICs, such as wavelength converters, 2Rdevices, modulators, etc. simply do not have monitoring capability. Inorder to test them, one must attach optical inputs and outputs, alignthese test inputs and outputs, and deal with sorting out the differencebetween input and output signal power attenuation due to internalproblems and that due to misalignment of the test probes or lossesthrough the test probe interfaces to the PIC.

Further exacerbating the problem is that one simply cannot assume agiven PIC is fully operable. The plain fact is that there are few, ifany, commercially available PIC devices that actually deliver theirstated specifications. Generally some monitoring is needed, at themanufacturing as well as operational stages to tractably utilize thesedevices.

What is needed in the art is an efficient method for monitoring PICdevices.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purposes of illustrating the various aspects of the invention,there are shown in the drawings forms that are presently preferred, itbeing understood, however, that the invention is not limited to theprecise arrangements and instrumentalities shown.

FIG. 1 depicts an exemplary PIC with redundancy;

FIG. 2 depicts modifications of the device of FIG. 1 according to thepresent invention;

FIG. 3 shows the passive MMI portion of the device of the presentinvention with various possible splitting of input power;

FIG. 4 shows the input signal lightpaths for the device of FIG. 2according to the present invention;

FIG. 5 shows the CW input lightpaths for the device of FIG. 2 accordingto the present invention; and

FIG. 6 depicts the lightpaths of FIGS. 4 and 5 superimposed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The above-described problems of the conventional art are solved inaccordance with the method and apparatus of the present invention. Anovel method and apparatus for internal monitoring and fault isolationin photonic integrated circuits (PICs) is presented.

Before one or more embodiments of the invention are explained in detail,it is to be understood that the invention is not limited in itsapplication to the details of construction or the arrangements ofcomponents set forth in the following description or illustrated in thedrawings (the terms “construction” and “components” being understood inthe most general sense and thus referring to and including, inappropriate contexts, methods, algorithms, processes and sub-processes).The invention is capable of other embodiments and of being practiced orbeing carried out in various ways. Also, it is to be understood that thephraseology and terminology used herein is for the purposes ofdescription and should not be regarded as in any way limiting.

To illustrate the method and apparatus of the present invention, an AO2R(all optical regeneration and reshaping)/Wavelength Conversion devicewill be used as an exemplary PIC. Such a device is depicted in FIG. 1,and is the subject of a pending US Patent Application under commonassignment herewith, and thus not prior art hereto, entitled “REDUNDANTPATH ALL-OPTICAL REGENERATION, RESHAPING AND WAVELENGTH CONVERSION FORENHANCED YIELD” filed on May 15, 2002, with David Lidsky and JithamithraSarathy as Applicants (the “AO2R Patent”) (U.S. application Ser. No.10/147,333, now issued U.S. Pat. No. 7,203,427).

As described in the AO2R Patent, due to inherent redundancy of thecircuit of FIG. 1, there are numerous possible circuit points at whichto place the input data signal, the input CW light source, and theoutput signal. With reference to FIG. 1, for the purposes ofillustration, an arbitrary placing of the “dirty” input data signal at101, the CW input at 102 and the “clean” output at 103 has been made.Given this placing, one of the SOAs from the bottom pair needs to bebiased for destructive interference relative to one SOA from the toppair by externally electrically biasing one of the lower SOAs. Thus, thechosen top SOA will undergo signal induced phase shift upon receipt ofan incoming “1” or “high” signal. With reference to FIG. 1, again,arbitrarily chosen for illustration purposes, SOA B1 120 is the chosentop SOA, and SOA B2 121 is the chosen bottom SOA. Similar numericaldesignations are used throughout this application for the elementscorresponding to those of FIG. 1; the only change is with the hundredsplace digit. Thus the CW input in FIG. 4 is designated as “402”, etc. .. .

Given these choices for inputs, outputs, and signal paths, there arecertain unneeded optical pathways, such as path segments 160 and 161.This is because if the input signal enters at 101 and flows through B1120 to induce phase modulation, neither the portion of the input signal101 nor the portion of CW signal 102 traveling through segment 160 hasany function. Similarly, if B2 121 is used as the externallyelectrically biased SOA, then the CW signal 102 traveling throughsegment 161 also has no function. In fact, these superfluous signalswould normally be absorbed by the non signal path SOAs, via appropriatebiasing of these SOAs or other means.

The present invention uses these non-signal path SOAs as photodetectors,and in so doing allows a PIC device is to be fabricated with built-ininternal power monitors, as shown in FIG. 2, where unneeded waveguidesegments 160 and 161 from FIG. 1 have been removed, and SOAs 281, 282,283 and 284 have been modified to function as photodetectors.Alternatively, if a PIC device does not have inherent redundancy, thereverse process can be implemented at the design level. Redundantpathways can be added to the design and photodetectors placed therein toimplement the method of the present invention.

With reference to FIG. 2, collectively, the integrated monitors willhave the ability to isolate faults to a single source: either (a) theincoming line signal 201 (the “dirty” signal to be regenerated &reshaped in the AO2R exemplary PIC device); (b) the incoming CW signal(the clean reference signal); or an internal chip (one of the SOAs).

In general SOAs can be converted to photodetectors (PD), either byreversing the current bias or by modifying design.

Once a given SOA is to operate as a photodetector, there is no reason tocontinue to route one-half of the optical signal to it. Thus, forreasons of efficiency, with reference to FIG. 2, MMIs whose outputs arethe PDs, i.e. 290, 291, 292, and 293 could have their splitting ratiomodified from 50:50 to 90:10, where 10% goes to the downstream PD. FIG.3 depicts MMIs having a 50:50 splitting ratio, and corresponding MMIswith a 90:10 splitting ratio.

To make the analysis for fault isolation, the PDs can be connected toTrans-Impedance Amplifiers and the light detected at the PD therebymeasured. If the light detected at the PD is less than a pre-setthreshold, a fault is declared.

The following advantages are offered by internal faultisolation/monitoring according to the method of the present invention:

Low cost.

Small Size.

Integrated design.

Fault isolation to a single source.

E.g., in the exemplary circuit of FIG. 2, the input path through PDs 283and 284 is distinct from the CW input path through PDs 290 and 291, thusallowing fault isolation to be pinpointed to either the “dirty” input.

Accurate (No external components).

Flexible Design (The Detection Threshold can be set by the user).

FIGS. 4-6 each depict the exemplary device of FIG. 2. FIGS. 4 and 5illustrate the lightpaths for the “dirty” input data signal and the CWreference signal respectively.

With reference to FIG. 4, the light gray waveguides represent thelightpaths for the input data signal 401. As can be seen with referenceto FIG. 4, there are two photodetectors involved in fault isolationrelative to this input data signal 401, PD#1 482 and PD#2 481. As thelegend at the bottom of FIG. 4 states, if the power loss is detected byboth PD#1 482 and PD#2 481 then there is a fault on the input 401. Ifthe power loss is detected only by one of these two photodetectors thentwo situations are possible. If a power loss is detected by PD#2 481 andnot PD#1 482, then the input signal is good and the problem must liewith the only active device in line between the input signal 401 andphotodetector PD#2 481; hence the fault lies with SOA B1 420. If a powerloss is detected by PD1 and not PD2, no conclusions can be drawn. If nopower loss is detected by either PD#1 482 or PD#2 481, then there is nofault whatsoever. The various possible combinations of faults detectedat photodetectors PD#1 and PD#2 are summarized in Table 1 below.

TABLE 1 Input Fault Isolation PD#1 PD#2 Fault 0 0 None 1 0 N/A 0 1Device (B1) 1 1 Input

FIG. 5 depicts the lightpaths used in monitoring the CW input 502.Because it is difficult to display colors of gray crosshatching has beenused to denote the lightpaths of the CW signal 502. As can be seen fromFIG. 5, the CW signal 502 enters through an initial SOA 540 and fromthere passes into an MMI 545 which splits the signal into two differentparts. MMI 545 splits the input power equally, i.e. in a 50:50 ratio,inasmuch as each output arm is recombined via SOAs 520 and 521 forconstructive or destructive interference as more fully described in theAO2R Patent. The portion of the CW input signal 502 that can besubjected to internal monitoring is carried through MMIs 548 and 547with reference to FIG. 5. It is these MMIs that can be set to anon-equal input power split, such as 90:10, tapping the smaller fractionof their input power for the monitoring output arm, as described above.As can be seen, PD#4 583 and PD#3 584 are involved in monitoring the CWinput signal 502. As the legend on the bottom of FIG. 5 indicates, if apower loss is detected by both PD#4 583 and PD#3 584, it can beconcluded that there is a fault with the CW input signal 502. If a powerloss is detected by PD#3 584 only, then it could be concluded that theonly in line active device, which is SOA B2 521, is the source of thefault. If a power loss is detected by PD#4 583 only and not PD#3 584,nothing definitive can be concluded. The various possible results andtheir meanings are summarized in Table 2 below.

TABLE 2 CW Fault Isolation PD#4 PD#3 Fault 0 0 None 1 0 N/A 0 1 Device(B2) 1 1 CW

FIG. 6 repeats and superimposes the information contained in FIGS. 4 and5, allowing the viewer to see the entire fault isolation systemsynoptically. While the example of the PIC device of FIG. 2 has beenused to illustrate the present invention, as described above any opticalintegrated circuit could be enhanced by the method and apparatus of thepresent invention simply by adding redundant pathways for the opticalsignals desired to be monitored and placing photodetectors in suchpathways.

As well, a photonic integrated circuit or PIC does not need to containMMIs in order for the method of the present invention to be applicable.The present invention is intended to be applied to any type or method ofsignal tapping for monitoring purposes such as, for example, directionalcouplers.

Finally, it is also possible to implement the method of the presentinvention where signals which are desired to be monitored co-propagatethrough a given detecting device. In the example described above, therewas no photodetector whose input was more than one signal. Withreference to FIG. 2, the photodetectors on the top of the figure wereused to monitor the input signal 201 and the photodetectors on thebottom of the figure were utilized to monitor the input CW signal 202. Acircuit could just as well be created such that photodetectors receivelight from two different sources which are desired to be monitored. Insuch case, there simply needs to be some type of filtration or selectionof the various co-propagating signals so that a given input could beisolated.

While the above describes the preferred embodiments of the invention,various modifications or additions will be apparent to those of skill inthe art. Such modifications and additions are intended to be covered bythe following claims.

1. A photonic integrated circuit having a plurality of circuitcomponents, comprising: an MMI for splitting signal power passingtherethrough among first and second optical pathways coupled to firstand second outputs, respectively, of the MMI, thereby directing firstand second percentages of the signal power along the first and thesecond optical pathways, respectively; and a photodetector integratedinto the photonic integrated circuit and coupled to said first opticalpathway for measuring a signal power level on said first opticalpathway.
 2. The photonic integrated circuit of claim 1 wherein thephotodetector is operable to compare the measured signal power level toa detection threshold.
 3. The photonic integrated circuit of claim 13wherein the photodetector is operable to signal a fault if the measuredsignal power is less than the detection threshold.
 4. The photonicintegrated circuit of claim 12 wherein the second optical pathway iscoupled to at least one signal path of the photonic integrated circuit.5. The photonic integrated circuit of claim 12 wherein the firstpercentage is between 10% and 50%.